Also called:
hydrophobia or lyssa
Key People:
Louis Pasteur
Sir Victor Horsley

News

Vaccination of sambar deer, zookeepers begins at city zoo Mar. 12, 2025, 2:30 AM ET (The Hindu)

rabies, acute, ordinarily fatal, viral disease of the central nervous system that is usually spread among domestic dogs and wild carnivorous animals by a bite. All warm-blooded animals, including humans, are susceptible to rabies infection. The virus, a rhabdovirus, is often present in the salivary glands of rabid animals and is excreted in the saliva; thus, the bite of the infected animal introduces the virus into a fresh wound. Under favourable conditions, the virus propagates along nerve tissue from the wound to the brain and becomes established in the central nervous system. After a time it spreads via nerves to the salivary glands, where it frequently produces a foaming at the mouth. The disease develops most often between four and six weeks after infection, but the incubation period may vary from 10 days to eight months.

Rabies virus travels quickly in a bitten animal (e.g., raccoons, skunks, bats, foxes, dogs, and cats, among other smaller animals) from the bite to the central nervous system. The disease often begins with excitation of the central nervous system expressed as irritability and viciousness. A rabid animal is most dangerous during the early stages of the disease because it appears to be healthy and may seem friendly but will bite at the slightest provocation. Wild animals that appear to be tame and that approach people or human habitations in the daytime should be suspected of having rabies.

Infected dogs usually show a short excitation phase that is characterized by restlessness, nervousness, irritability, and viciousness and is followed by depression and paralysis. After a few days they are unable to bite any more because the muscles of the throat are paralyzed; they seek only a quiet place to hide and die from the rapid spread of paralysis. Sudden death without recognizable signs of illness is also not uncommon. Dogs that develop the predominantly excited type of rabies invariably die of the infection, usually within three to five days after the onset of symptoms. Those that develop the paralytic type of rabies without any evidence of excitation or viciousness may recover on rare occasions. Paralysis of the “voice” muscles in rabid dogs may produce a characteristic change in the sound of the bark.

Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph.
Britannica Quiz
44 Questions from Britannica’s Most Popular Health and Medicine Quizzes

Rabies in humans is similar to that in animals. Symptoms include depression, headache, nausea, seizures, anorexia, muscle stiffness, and increased production of saliva. Abnormal sensations, such as itching, around the site of exposure are a common early symptom. The muscles of the throat become paralyzed so that the person cannot swallow or drink, and this leads to a dread of water (hydrophobia). The mental state of a person infected with rabies varies from maniacal excitement to dull apathy—the term rabies means “madness”—but soon the person falls into a coma and usually dies in less than one week owing to cardiac or respiratory failure. Sometimes rabies is characterized by paralysis without any evidence of excitation of the nervous system. In such cases the course of the disease may be prolonged to a week or more.

There is no cure for rabies. The incubation period (the time that elapses between the bite and the first symptom) is usually one to three months but in rare cases has been as long as several years. This provides a chance to interrupt the otherwise inevitable progress of the infection. The bite should be washed immediately because much, if not all, of the virus can be thus removed. The bitten patient should then receive a dose of antirabies serum. Serum is derived from horses or humans that have been immunized with attenuated rabies virus; it provides the patient with already prepared antibodies against the rabies antigen. The treatment is effective if given within 24 hours after exposure but has little, if any, value if given three or more days after infection by rabies.

Active immunization with rabies vaccine should also be initiated to allow the patient’s body to make its own antibody. The safest and most effective vaccines are human diploid cell vaccine (HDCV), purified chick embryo cell culture (PCEC), and rabies vaccine adsorbed (RVA). With older vaccines, at least 16 injections were required, whereas with HDCV, PCEC, or RVA, 5 are usually sufficient. Persons at risk of rabies by virtue of occupation (e.g., veterinarians) or travel to endemic areas should receive rabies vaccine as a form of preexposure prophylaxis.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn.
Britannica Chatbot logo

Britannica Chatbot

Chatbot answers are created from Britannica articles using AI. This is a beta feature. AI answers may contain errors. Please verify important information using Britannica articles. About Britannica AI.
Also called:
zoonosis

News

One lakh women farmers get training on managing zoonotic diseases Mar. 8, 2025, 6:34 AM ET (The Hindu)
Study explains why tecovirimat fails against some mpox strains Feb. 13, 2025, 9:57 PM ET (News-Medical)

zoonotic disease, any of a group of diseases that can be transmitted to humans by nonhuman vertebrate animals, such as mammals, birds, reptiles, amphibians, and fish. A large number of domestic and wild animals are sources of zoonotic disease, and there are numerous means of transmission. Public health veterinarians have a critical role in zoonotic disease surveillance, prevention, and control, but risk reduction increasingly requires multidisciplinary teams and a unified concept of medicine in humans and other animal species.

Zoonotic disease classification

All classes of disease agents cause zoonotic disease, including bacteria, viruses, parasites, and fungi. Although zoonotic diseases can be classified according to their infectious agents, they also can be subdivided into those diseases that are transmitted from nonhuman animals to humans or from humans to nonhuman animals. Examples of the complex pathways of transmission among zoonotic diseases include the spread of Mycobacterium tuberculosis from humans to cattle and elephants and the transmission of methacillin-resistant Staphylococcus aureus (MRSA) from humans to horses and back to humans. Some diseases are considered to be zoonotic even though they are rarely transmitted between nonhuman animals and humans; an example is foot-and-mouth disease in cattle.

Zoonotic diseases also can be classified according to their life cycle. Diseases that are transmitted directly (e.g., through direct contact or a mechanical vector) and that are maintained in nature in a single vertebrate host species are known as orthozoonoses; an example is rabies, which is maintained by canids. Cyclozoonoses, such as echinococcosis, require more than one vertebrate host for development. Metazoonoses require both a vertebrate host and an invertebrate host; an example is trypanosomiasis. Zoonotic diseases that require a vertebrate host and another type of environmental reservoir (e.g., food or soil) are known as saprozoonoses. Listeriosis and histoplasmosis are examples of saprozoonoses.

Encyclopaedia Britannica thistle graphic to be used with a Mendel/Consumer quiz in place of a photograph.
Britannica Quiz
44 Questions from Britannica’s Most Popular Health and Medicine Quizzes

Populations at increased risk

Any person who comes into contact with an infected animal, vector, or contaminated area can become infected with a zoonotic disease. However, the risk of acquiring disease, the clinical signs of disease, and the risk of death are not uniformly distributed across individuals. The proportion of people who remain asymptomatic and the case fatality rate (proportion of ill persons who die) vary with certain risk factors. For example, age often is associated with disease severity. Of those infected with Escherichia coli O157:H7 from contact with animals or their environment, very young children and the elderly are more likely to develop potentially fatal hemolytic uremic syndrome (HUS) than are older children and healthy adults. By contrast, hantavirus appears to be especially deadly among fit young adults and middle-aged individuals, possibly owing to the increased likelihood of those individuals’ coming into contact with the infectious agent.

The risk of becoming infected with a zoonotic disease is increased in persons affected by immunosuppression from a preexisting disease or medication. For example, cryptosporidiosis caused by Cryptosporidium parvum, which is transmitted to humans following contact with calves, their manure, or manure-contaminated objects or food, can occur as a coinfection with acquired immunodeficiency syndrome (AIDS). Normally a self-limiting disease, in those with AIDS cryptosporidiosis can cause serious illness, sometimes ending in death. Persons without a functioning spleen have an increased risk of illness and death from Capnocytophaga canimorsus infection, which can be acquired through contact with cats or dogs (particularly through dog bites). Persons who take chloroquine for malaria prophylaxis concurrently with rabies preexposure immunizations are less likely to develop a sufficient immunologic response to survive a rabies exposure.

Other populations at risk include those who are cognitively impaired; such individuals, for example, may not be able to recognize or report bites from rabid bats. Pregnant women are at risk of fetal congenital malformations with lymphocytic choriomeningitis virus (LCMV) infection. Solid-organ transplant recipients have died from rabies and LCMV infections transmitted from donors.

Zoonotic disease control

Zoonotic diseases are difficult to control, particularly because of their animal reservoirs. Indeed, unlike diseases such as smallpox and polio, most zoonotic diseases cannot be eradicated through intensive human vaccination campaigns. Their successful control relies instead on strategies aimed at reducing the burden of disease among wild animals. In the case of rabies, for example, the distribution of baits containing oral rabies vaccine has led to the near-elimination or eradication of variant rabies (e.g., the Arctic fox and red fox variants) from regional wildlife reservoirs (e.g., foxes and raccoons).

Are you a student?
Get a special academic rate on Britannica Premium.

Zoonotic disease risk is increased when humans live in close proximity to domestic animals such as poultry and livestock. Although the practice allows for the efficient use of limited land resources and constant care and protection of the animals, it also increases the risk of humans’ becoming infected with disease agents such as HPAI (highly pathogenic avian influenza—e.g., H5N1 virus). Pets, which often live in human homes, are common sources of zoonotic disease. For example, Salmonella infections (sometimes with multidrug-resistant strains) can occur as a result of contact with pet reptiles and amphibians (e.g., turtles, iguanas, and snakes), exotic pets (e.g., hedgehogs and sugar gliders), pocket pets (e.g., hamsters, mice, and rats), pet birds (e.g., chicks and ducklings), and dogs and cats. Pet treats and other pet-associated environmental factors may also be sources of Salmonella.

Even in areas with separation between human homes and animal facilities, zoonotic diseases still pose a risk to humans, largely because of human contact with animals. Large E. coli O157:H7 outbreaks, for example, have been associated with dairy farms, children’s day camps conducted in farm settings, social events in buildings previously used for animal exhibitions, fair petting zoos, and contaminated fair water systems. Critical control methods in homes and public settings include animal management to reduce disease burden, management of animal and human contacts, and education to reduce exposure (particularly by handwashing).

Limiting contact between humans and wild animals is critical to reducing the risk of zoonotic disease transmission. Many human rabies deaths are due to bites from bats, frequently in home settings. Although the human immunodeficiency virus (HIV), which causes AIDS, is not zoonotic, it is thought to have evolved from similar monkey viruses that jumped to humans through the practice of hunting and consuming bush meat (monkeys). Contact with rodent feces is associated with hantavirus infection, and plague infection is associated with activities that bring people into contact with wild rodents and their fleas. The risk of zoonotic disease in humans can be further reduced by limiting contact between wild and domestic animals.

Because zoonotic disease agents can be found in humans, animals, the environment, and vectors, management requires the collaboration of many types of health and disease-control specialists. Disease control may include vector-control programs for ticks, fleas, or mosquitoes, and environmental cleanup or protection may be required to address disease agents that remain viable from days to years on surfaces, in soils, or in the water. In most state health agencies, public health veterinarians are available to assist in disease-control coordination.

The control of zoonotic disease has been challenged by increases in human-associated movement of animals. For example, raccoon rabies was introduced to the central east coast of the United States by deliberate human movement of raccoons from the southeastern United States for hunting purposes. The disease subsequently spread throughout the east coast and into the midwestern United States and parts of Canada. Mpox (infection with monkeypox virus) of African rodents imported for the pet trade led to cases of the disease in pet prairie dogs and humans, resulting in the introduction of restrictions on the pet prairie dog trade. The spread of H5N1 bird flu in Asia, the Middle East, and Europe likely was the result of human movement of infected domestic birds as well as wild bird migration.

Zoonotic diseases continue to be of significant concern for public health. In the early 21st century, an estimated 60 percent of novel human pathogens were zoonotic in origin, and increasing numbers of zoonotic diseases were spreading into areas where they previously had not occurred. In addition, several zoonotic disease agents were identified as candidates for use in bioterrorism attacks.

Millicent Eidson The Editors of Encyclopaedia Britannica
Britannica Chatbot logo

Britannica Chatbot

Chatbot answers are created from Britannica articles using AI. This is a beta feature. AI answers may contain errors. Please verify important information using Britannica articles. About Britannica AI.